Enquiry into the evolution of ageing, or aging, aims to explain why a detrimental process such as ageing would evolve, and why there is so much variability in the lifespans of organisms. The classical theories of evolution (mutation accumulation, antagonistic pleiotropy, and disposable soma) suggest that environmental factors, such as predation, accidents, disease, starvation, ensure that most organisms living in natural settings will not live until old age, and so there will be very little pressure to conserve genetic changes that increase longevity. Natural selection will instead strongly favor genes which ensure early maturation and rapid reproduction, and the selection for genetic traits which promote molecular and cellular self-maintenance will decline with age for most organisms. Theories and hypotheses The beginning August Weismann was responsible for interpreting and formalizing the mechanisms of Darwinian evolution in a modern theoretical framework. In 1889, he theorized that ageing was part of life's program to make room for the next generation in order to sustain the turnover that is necessary for evolution. The idea that the ageing characteristic was selected (an adaptation) because of its deleterious effect was largely discounted for much of the 20th century, but a theoretical model suggests that altruistic ageing could evolve if there is little migration among populations. Weismann later abandoned his theory and after some time followed up with his "programmed death" theory. Natural selection is a process that allows organisms to better adapt to the environment, it is the survival of the fittest which are predicted to produce more offsprings. Natural selection acts on life history traits in order to optimize reproductive success and lifetime fitness. Fitness in this context refers to how likely an organism is to survive and reproduce. It is based on the environment and is also relative to other individuals in the population. Examples of life history traits include; age and size at first reproduction, number of size and offsprings produced, and the period of reproductive lifespan. Organisms put energy into growth, reproduction, and maintenance by following a particular pattern which changes throughout their lifetime due to the trade-offs that exist between the different energy allocations. Investment in current vs future reproduction, for example, comes at the expense of the other. Natural selection, however is not so effective on organisms as they age. Mutation accumulation (MA) and antagonistic pleiotropy (AP) are two factors which contribute to senescence. Both MA, and AP contribute to